Andean Geology ISSN: 0718-7092 [email protected] Servicio Nacional de Geología y Minería Chile González, Javier; Niemeyer, Hans; Benedetto, Juan L.; Brussa, Edsel D. The Ordovician Quebrada Grande Formation, Cordón de Lila (Antofagasta Region, northern Chile): stratigraphic and paleogeographic significance Andean Geology, vol. 34, núm. 2, julio, 2007, pp. 277-290 Servicio Nacional de Geología y Minería Santiago, Chile Available in: http://www.redalyc.org/articulo.oa?id=173918417006 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative The Ordovician Quebrada Grande Formation, Cordón de Lila (Antofagasta Region, northern Chile): stratigraphic and paleogeographic significance Javier González Departamento de Ciencias Geológicas, Universidad Católica del Norte, Hans Niemeyer Av. Angamos 0610, Antofagasta, Chile [email protected] [email protected] Juan L. Benedetto CONICET, Centro de Investigaciones Paleobiológicas, Universidad Nacional de Córdoba, Av. Vélez Sarsfield 299, 5000, Córdoba, Argentina [email protected] Edsel D. Brussa CONICET, Departamento de Ciencias Naturales, Universidad Nacional de La Pampa, Uruguay 151, 6300, Santa Rosa, Argentina [email protected] ABSTRACT The Cordón de Lila is located immediately to the south of the Salar de Atacama, in northern Chile. The geology of the Cordón de Lila is characterized by extensive outcrops of Early Paleozoic volcanic and sedimentary rocks (Cordón de Lila Igneous and Sedimentary Complex; CISL) that form the ‘Arco magmático occidental’ (AMO) which is intruded by a multiple suite of Middle Ordovician to Lower Silurian granitoids. In this contribution we report the results of a sedimentological and paleontological study of a recently discovered 1,600 m thick marine Ordovician sedimentary sequence (Quebrada Grande Formation) which unconformably overlies the CISL. The Quebrada Grande Formation comprises three mayor facies associations. In stratigrafic order, these facies associations are: matrix-supported conglomerates (1), interbedded sandstones and siltstones (2) and clast-supported conglomerates (3). Facies association (1) is interpreted as debris flow deposits accumulated in a proximal marine fan-delta setting while the overlying sandstones and siltstones facies (2) represent open-marine platform deposits. Clast-supported conglomerates (3) are the record of high-energy sedimentary episodes during which coarse sediments reached the distal part of the fan-delta. The detrital fraction of the Quebrada Grande Formation is derived from the erosion of the Cambrian?-Lower Ordovician? CISL volcanic arc, the plutonic roots of the arc and its continental basement. The age of this formation is well constrained by the occurrence of brachiopods (Paralenorthis sp., Monorthis transversa Benedetto, and Mollesella? sp.) near the base and graptolites (Tetragraptus sp. and Cryptograptus? sp.) towards the middle of the succession. Both brachiopods and graptolites indicate an Arenigian to early Llanvirnian (Darriwilian) age. The brachiopod assemblage from Cordón de Lila displays affinities with the Famatina basin fauna of western Argentina. The absence of taxa in common between the AMO and the nearby ‘Faja Eruptiva de la Puna Occidental’ (FEPO) of NW Argentina may result from some kind of environmental barrier or some geographic separation from the Gondwana margin. According to the latter hypothesis, the Quebrada Grande succession developed on the allochtonous Arequipa-Antofalla Terrane. Key words: Ordovician, Brachiopods, Graptolites, Arequipa-Antofalla Terrane, Cordón de Lila, northern Chile. Revista Geológica de Chile, Vol. 34, No. 2, p. 277-290, 8 Figs., July 2007. 278 THE O RDOVICIAN Q UEBRADA G RANDE F ORMATION, CORDÓN DE L ILA (ANTOFAGASTA R EGION, NORTHERN C HILE)... RESUMEN La Formación Quebrada Grande, del Ordovícico, Cordón de Lila (Región de Antofagasta, norte de Chile): significado estratigráfico y paleogeográfico. En el Cordón de Lila, ubicado al sur del Salar de Atacama, en la Región de Antofagasta, aflora una sucesión volcánica y sedimentaria del Paleozoico más temprano (Complejo Ígneo y Sedimentario del Cordón de Lila; CISL) que forma el ‘Arco magmático occidental’ (AMO), intruida por granitoides del Ordovícico medio-Silúrico inferior. En esta contribución se presenta un análisis sedimentológico y paleontológico de la recientemente reconocida Formación Quebrada Grande, una secuencia marina de ca. 1.600 m de espesor, que cubre, mediante discordancia angular, al CISL, y en la cual es posible definir tres asociaciones de facies que, en orden estratigráfico, son: 1) conglomerados matriz soportados, 2) intercalaciones de areniscas con lutitas y 3) conglomerados clastosoportados. La fracción detrítica deriva principalmente de la erosión del arco volcánico representado por el CISL (¿Cámbrico-Ordovícico inferior?), además de rocas plutónicas que formaron la raíz del arco, y de rocas metamórficas provenientes de su basamento continental. La facies (1), que constituye la parte inferior de la formación, se interpreta como el producto de flujos de detritos en posición proximal de un ‘fan delta’ marino. Las areniscas y lutitas (2), de la parte media de la formación, representan un ambiente de plataforma marina abierta. Los conglomerados clasto-soportados (3) corresponderían a flujos de alta energía que alcanzaron las partes más distales del delta. La edad de la Formación Quebrada Grande se puede precisar por la presencia de braquiópodos (Paralenorthis sp., Monorthis transversa Benedetto y Mollesella? sp.) en la base y de graptolitos (Tetragraptus sp. y Cryptograptus? sp.) en el medio de la sucesión, que indican el lapso Arenigiano-Llanvirniano inferior (Darriwiliano). La asociación de braquiópodos del Cordón de Lila presenta afinidades con las faunas de la cuenca de Famatina del oeste de Argentina. La ausencia de taxones comunes entre el AMO y la ‘Faja Eruptiva de la Puna Occidental’ (FEPO) del noroeste de Argentina podría deberse a algún tipo de barrera ambiental o a una separación geográfica respecto del margen de Gondwana. De acuerdo a la última hipótesis, la sucesión de Quebrada Grande podría haberse desarrollado sobre el Terrane alóctono de Arequipa-Antofalla. Palabras claves: Ordovícico, Braquiópodos, Graptolitos,‘Terrane’ de Arequipa-Antofalla, Cordón de Lila, norte de Chile. INTRODUCTION The Cordón de Lila is located to the south of the discussion of the depositional environment and the Salar de Atacama in northern Chile (Fig. 1). It is well paleogeographic significance of the Quebrada known by the occurrence of important outcrops of Grande Formation. Lower Paleozoic sedimentary, volcanic succes- sions and intrusive rocks (Mpodozis et al., 1983; Niemeyer, 1989; Damm et al., 1990; Pérez et al., 2006; Navarro et al., 2006). The study of these units is key for the understanding of the stratigraphic, tectonic, and paleogeographic relationships be- tween the Ordovician successions of the Argentina- Chilean Puna (Benedetto, 2001). The purpose of this paper is to discuss the age and paleobiogeo- graphic affinities of the brachiopod and graptolite fauna of Ordovician age recently found in one of the Early Paleozoic successsions (the Quebrada Grande Formation), that crops out at Cordón de Lila (Benedetto et al., in press). In this paper, we also describe, the main facies association which, FIG. 1. Location map of Cordón de Lila in the southern part of Salar together with the study of the fossil fauna, allows the de Atacama. J. González, H. Niemeyer, J.L.Benedetto, E.D. Brussa 279 PALEOZOIC GEOLOGY OF THE CORDÓN DE LILA The distribution of the Paleozoic lithostrati- of conglomerates, sandstones and siltstones which graphic units at Cordón de Lila is shown in figure was referred to as Quebrada Grande Formation by 2. The oldest stratified rocks exposed in the area González et al. (2006). belong to the ‘Complejo Ígneo-Sedimentario del The Quebrada Grande Formation is intruded by Cordón de Lila’ (CISL; Niemeyer, 1989). The lower the Quebrada Ancha Porphyry, which contains two thirds of this approximately 3,000-m-thick conglomerate xenoliths coming from the Quebrada succession consist of basalts interbedded with Grande Formation. The porphyry is in turn uncon- turbidites, whereas the upper third is formed mainly formably overlain by Lower Silurian quartzitic and by rhyolitic and rhyodacitic volcanic flows with few calcareous sandstones of the Quebrada Ancha fine grained sediment interbeds (Niemeyer, 1989; Formation (Navarro et al., 2006). All these rocks are Pérez et al., 2006). The age of this complex is not intruded by huge volumes of granitic rocks of well constrained and has been tentatively assigned Middle-Upper Ordovician to Lower Silurian age to the Cambrian?-Lower Ordovician? on the basis (Mpodozis et al., 1983; Damm et al. 1990). The of their stratigraphic relationships (Niemeyer, 1989; plutonic rocks are covered by marine quartzarenites Damm et al., 1990). The CISL is unconformably and conglomerates of the Devonian Lila Formation overlain by a marine succession of about 1,600 m (Breitkreuz, 1986). FIG. 2. Geology of the Cordón de Lila. a- Stratigraphic section in the Quebrada Grande Formation; b- Faults; c- Stratified Paleozoic units: (1) ‘Complejo Ígneo-Sedimentario del Cordón de Lila’, CISL: basaltic and rhyodacitic lavas interbedded with